Synchronisation of frame transmissions in a telecommunications network

ABSTRACT

The invention concerns a method for synchronising clocks of base transceiver stations in a telecommunications system and a mobile communications system. According to the invention, in either some or in all base station sites of the telecommunications network a location measurement unit is installed, which receives an accurate clock including an extra-system signal. The clock information is transmitted further to base transceiver stations, whose frame transmissions are synchronised in accordance with the clock information. By using the implementation in accordance with the invention both economic and functional advantages are achieved, e.g. the capacity can be increased by up to 50-300%, depending on the manner of implementation.

FIELD OF THE INVENTION

The invention relates to synchronisation of a mobile network, especiallyto synchronisation of transmissions over the air interface.

TECHNICAL BACKGROUND

At the present time special attention is paid in several differentcountries to the satellite-based localisation of mobile stations and tovarious services relating to localisation. For example, in the UnitedStates of America the authorities have established a time limit, bywhich mobile stations must include a localisation system, with the aidof which a call received by an emergency station can be localised withan accuracy of approximately one hundred meters. Mobile stations with anintegrated GPS-based (Global Positioning System) localisation system arealready commercially available.

In practice, such a GPS receiver is integrated into the mobile station,which receives signals transmitted by satellites and which calculatesits location with the aid of information contained in the signal. TheGPS satellite transmits two different carrier wave frequencies. Twopseudo random signals (a C/A code and a P code intended for militaryuse) and a navigation message are modulated into the carrier waves. Thecode messages do not contain actual information, but they are pseudorandom sequences including a +1 and a −1 space and they are used formodulating the carrier wave. In addition, such a navigation message ismodulated into the carrier wave, which contains information related todetermination of the location. The length of its frame is 1500 bits, andthe frame is divided into five sub-frames, all of an equal length. Theframe includes e.g. clock correction information, check bits,information on the age of data, on the orbits of satellites and on thelocation of satellites at different times.

Continuous three-dimensional localisation requires no less than foursimultaneously visible satellites. The determination of location isbased on a determination of the distance between receiver and the groupof satellites. The location can be determined with the aid of at leastthree satellites, whereby the fourth is mainly needed to eliminatedeviations caused by clock errors.

Since the signal transmitted by a satellite is accompanied byinformation on the time of departure of the signal, the receiver is ableto calculate the signal's transit time from the difference between itsown clock time and the satellite's clock time. The distance of thesatellite can hereby be determined, by multiplying the signals transittime by the signal's velocity, that is, by the velocity of light.

There are several different kinds of GPS signal receivers. Thedifferences relate e.g. to whether the receiver is a single-frequencyreceiver or a two-frequency receiver, whether the receiver observesseveral satellites constantly or each satellite alternately, and whatkind of code it identifies. The number of channels is an importantfeature of the receiver. This means how many satellites the receiver isable to observe simultaneously. The channels may be implemented eitherat equipment level or by software.

To allow as accurate a time determination as possible, the satelliteincludes a cesium clock, with which it takes more than two hours for onenanosecond to accrue. The clock error is taken into account bytransmitting the clock error information coded into the signal. Theclock of the receiver is usually a quartz crystal, which is lessaccurate than the atomic clock. With the quartz crystal it takes only asecond for a nanosecond error to accrue, and its errors are moredifficult to foretell. The clock error can be corrected, either byhaving the receiver update its clock when the error has grown to acertain predetermined value, or by using an external stable source offrequency.

Localisation of a mobile station does not require synchronised networks.Base transceiver stations are independent of each other, so the framestransmitted by them are not in synchrony with each other. In otherwords, base transceiver stations are not aware of the starting time ofthe frames they transmit. This drawback is due to the fact that noreliable reference clock has been available so far for implementingsynchronisation. In a non-synchronised network, the various frames willcause such interference to one another, which may at worst destroyseveral time slots. FIG. 1 illustrates the present situation, where basetransceiver stations located in the cells (cell 1-cell 5) of a cellularnetwork transmit frames over the air interface in a non-synchronisednetwork. As can be seen in the figure, the time slots TS1-TS8 of theframes are not mutually matched. It is impossible to know whichfrequencies are interfering with what, and the base station controlleris unable to calculate the mutual timings of the frames transmitted bythe cells.

The above-mentioned drawback can be reduced considerably, if the frametransmissions to be transmitted at the air interface are synchronised.In a synchronised network, no more than one time slot is lost in theworst case. FIG. 2 illustrates broadcasting of frames in a synchronisednetwork. When the frames are exactly matched, e.g. allocation ofchannels can be done exactly on a time slot basis.

For the localisation function, measuring equipment will be installed inthe future in the mobile network at the locations of base transceiverstations for localisation of mobile stations. In accordance with thepresent invention, these pieces of equipment can be utilised also forsynchronisation of frame transmissions at the air interface in a mobilenetwork. The solution is economically advantageous, because no separateequipment is required for the synchronisation only.

BRIEF SUMMARY OF THE INVENTION

The invention aims at synchronising frame transmissions over the airinterface in a mobile network. In practice, this means transmissionsbetween base transceiver stations and transmissions between a mobilestation and base transceiver stations.

The established aims are achieved in such a way that at the locations ofbase transceiver stations such a measuring unit is installed forlocalisation of the mobile station, which includes a receiver forreceiving a signal transmitted from a satellite. It is sufficient forthe location measurement unit to detect just one satellite orbiting theearth.

In accordance with a first embodiment of the invention, the locationmeasurement unit is installed at each base transceiver station site. Thelocation measurement unit synchronises is internal clock with the globalclock it receives from the satellite. A certain global moment in timerepresents a point of reference for the starting moment of the frameclock, that is, for the clock's edge. At the point of reference, theframe number is set at zero. The location measurement unit transmits theframe number thus defined and the frame clock to the base transceiverstation. The base transceiver station synchronises its own internalclock to correspond with the global time of the satellite. In thismanner, the internal clock of all base transceiver stations in thenetwork are made to show exactly the same time, whereby the frames canbe transmitted exactly at the same time from the different basetransceiver stations of the network.

This solution is simple to implement and the network synchronisationwhich can be achieved is exact. Economically, the implementation isexpensive, because it requires a considerable number of locationmeasurement units. Considered in the long term, the solution isadvantageous, since the synchronisation does not require separatepurchases of equipment just to do with the synchronisation, but theimplementation can use the same equipment as that which is used for thelocalisation of mobile stations.

In accordance with a second embodiment of the invention, the locationmeasurement unit is installed at some base transceiver station sites. Inthis implementation, the location measurement unit measures thesynchronisation difference of frame broadcasts from the base transceiverstation of its place of location and from adjacent base transceiverstations. It transmits the real time difference, which it has measured,along with time stamped frame numbers by way of the base transceiverstation to the base station controller. The base station controllercalculates the tuning values of the clock and changes the frame numberof each base transceiver station, so that each base transceiver stationwill use the correct frame number and so that the real time differencesbetween different base transceiver stations are minimised.

Since in this implementation there are considerably less locationmeasurement units, this solution in comparison with the previous one iseconomically more advantageous, considered in the short term. However,its manner of solution is more complicated and less accurate from theviewpoint of synchronisation.

With the implementation according to the invention, considerableadvantages are achieved both in economical and functional terms, e.g.the capacity can be increased by as much as 50-300%, depending on themanner of implementation.

LIST OF FIGURES

In the following, the invention will be described in greater detail withthe aid of the appended diagrammatic figures, of which

FIG. 1 shows frame transmissions of various cells in a non-synchronisedsystem,

FIG. 2 shows frame transmission of various cells in a synchronisedsystem,

FIG. 3 illustrates a location measurement unit in accordance with theinvention and its connection,

FIG. 4 shows an arrangement in accordance with the invention,

FIG. 5 illustrates a solution in accordance with a first embodiment ofthe invention,

FIG. 6 illustrates a solution in accordance with a first embodiment ofthe invention,

FIG. 7 shows an arrangement in accordance with a second embodiment ofthe invention,

FIG. 8 illustrates a solution in accordance with a second embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The mobile station localisation function in a mobile network can beimplemented with the aid of a Location Measurement Unit (LMU).

FIG. 3 illustrates diagrammatically a location measurement unit 301 andits connection to a Base Station (BTS) 302.

The location measurement unit includes e.g. an antenna 304 and a GPSreceiver for receiving GPS signals from satellites orbiting the earth.In addition, it includes a transceiver consisting of a transmitter TXand a receiver RX for data transmission over the air interface, acontrol unit CU, which performs calculation and controls the logic ofthe location measurement unit as well as the power supply. The outputgate of the location measurement unit is connected by a cable 303 to theinput gate of the base station's clock unit.

FIG. 4 illustrates a situation, wherein location measurement unit 301receives a GPS signal transmitted by satellite 402 orbiting the earth.From the message the location measurement unit separates a clock, withthe global time of which it synchronises its own internal clock. In asimplified manner the figure shows only one satellite, but normally theGPS receiver detects simultaneously at least four satellites orbitingthe earth. The GPS receiver integrated into the location measurementunit is fixed, so that its exact geographical location is known.Likewise, the location coordinates of each base station in the networkare known.

From the exact GPS time received from the satellites the locationmeasurement unit forms the GMS system's frame number FN and frame clockFCLK, which are relayed further to the base station 302. The basestation synchronises its own internal clock with the time transmitted bythe location measurement unit, whereby each base station of the networkcan transmit frames 401 over the air interface at the same moment t.

Synchronisation of the network may be implemented e.g. in such a waythat a location measurement unit is located at all base station sites,or alternatively so that a location measurement unit is located at somebase station sites only. These embodiments will be examined in thefollowing by way of examples.

First Embodiment of the Invention

FIGS. 4 and 5 illustrate an implementation of the invention wherein alocation measurement unit is installed at all base station sites. Inthis embodiment the location measurement unit functions as the masterclock of the base station site.

In FIG. 5, the location measurement unit 301 emulates a so-called talkbase station functioning as master in the synchronisation. The locationmeasurement unit and the base station are connected to each other by acable in the manner described in the foregoing. The certain GPS timereceived by the location measurement unit from the satellite representsa point of reference, from which the frame number begins, that is, FN=0,and the frame clock FCLK starts (the clock's edge). The locationmeasurement unit transmits a 24-bit long frame number and a frame clock(e.g. 217 Hz) to the base station. Besides the frame clock, also otherclock information may be transmitted when required, of which the TCLK(rimeslot Clock) synchronising the time slots of the frame and/or theSBCK (Serial Bus Clock) for use at the applicant's talk base stationsmay be mentioned as examples. The base station synchronises its clockaccording to the frame clock. Every location measurement unit and basestation in the network functions in a similar manner. Since the exactglobal GPS time is available to every base station in the network, thebase stations can transmit frames exactly at the same time, in otherwords, they can function in mutual synchrony. The Base StationController BSC 501 only manages phasing of the frames, e.g. by providingthe base station with synchronisation commands.

Since each base station in the network is sending frames in thesynchronised network at the same time, the base station controller isable to calculate on a time slot basis which channel ought to beallocated at each time. In practice, the aim is to allocate such afrequency for the mobile station, whose C/I, that is, Carrier overInterface ratio, is good enough. This means that the quality of speechor data transmission is good, in other words, the subscriber notices nointerference in the call.

In practical implementations, base stations of several sizes and modelsand installed at different times may be located side by side on the basestation site. Hereby it is unprofitable economically to install an ownlocation measurement unit separately at each base station. On the otherhand, it is more advantageous to chain the base stations with each otherand to install only one location measurement unit as the master clockfor the chained base stations.

FIG. 6 illustrates a situation, wherein there are several different basestations 601-603 on the base station site. Location measurement unit 301is the master clock of the base station site. Chaining of base stationsmeans that the output gate of the clock unit of the first base station601 is connected functionally by cable 303 to the input gate of theclock unit of the second base station 602 and, further, the output gateof the said clock unit is connected functionally by a cable to the inputgate of the clock unit of the third base station 603.

The location measurement unit transmits the frame number and clockinformation to the clock unit of the first base station. Thissynchronises itself according to the information it has received andtransmits the information forward to the following base station, etc. Ofcourse, the chaining will cause a delay in the clock. Since thedistances between base stations are short, that is, the connectingcables between them are short, the delays are rather small and it issimple to correct them. There are different methods for correctingdelays. One method is to correct a time delay in such a way that thetransmitting base station adds the correction value of the clock to theframe number and clock information. On the other hand, the correctionvalue may be stored in the memory of the receiving base station, fromwhich the receiving clock unit will pick the value and add it to theclock time obtained from the preceding base station.

The antenna 304 of the location measurement unit causes longer timedelays than those mentioned above, because its distance from the unit islonger than the lengths of base station cables. However, like theprevious delays, the one caused by the antenna can be calculated,because the parameters causing delays are known: The correction value ofthe delay caused by the antenna can be added to the frame number andclock information in the location measurement unit.

Second Embodiment of the Invention

With the aid of FIGS. 7 and 8 such an embodiment according to theinvention is studied in the following, wherein a location measurementunit is provided only on some base station sites. Since the embodimentrequires much less units, it is of course also much cheaper to implementthan the former. However, in a functional comparison with the former itis complicated and requires more calculation.

FIG. 7 illustrates a situation, wherein location measurement unit 301 islocated in connection with base stations 701, 703 and 704. Base stations702 and 705 have no location measurement unit. The base stations areconstantly broadcasting information about themselves on the BroadcastControl Channel BCCH. Mobile station 706 is connected over the radiopath with one of the base stations shown in the figure, e.g. with basestation 704. The base station controls the operation of the mobilestation e.g. by providing the synchronisation and by stating thetransmission power to be used by the mobile station. In state-of-the-artsolutions, a hierarchical or plesiochronous synchronisation is usuallyused, whereby the base station receives the synchronisation signal fromthe base station controller and the base station controller for its partreceives the synchronisation signal from the mobile services switchingcentre, etc. A public telephone system and radio system are usuallysynchronised at their topmost level as accurately as possible with anatomic clock. The synchronisation signal, which is e.g. a signal with anominal frequency of about 2 MHz, can be relayed from the base stationcontroller to the base station along a separate cable or over the radiopath. This method of synchronisation means that the base stations arenot exactly mutually synchronised, for which reason each base stationtransmits the own timing information of its cell on the above-mentionedBCCH channel.

The geographical locations of both the location measurement unit and thebase stations are accurately known. The location measurement unitmeasures the broadcasting of base stations in the same manner as themobile station does. Since the distance travelled by signals broadcastby base stations is accurately known, the location measurement unit isable from the measurement information to calculate the relativeso-called real time difference RTD of frame broadcasts by base stations.FIG. 8 illustrates a situation wherein location measurement unit 301broadcasts the real time difference and the time-stamped frame number tobase station 701. It broadcasts this information both to its own basestation and to adjacent base stations 702-705, which information isbroadcast further to base station controller 501. When the real timedifference RTD is combined with the global positioning system GPS, theAbsolute Time Difference ATD will result.

On the basis of information received from the location measurement unitsof base stations 701, 703 and 704 a calculation unit integrated into thebase station controller calculates clock tuning values for base stations702 and 705. In case the base station tuning is not correct, the basestation controller sends a command either to slow down or to speed upthe clock for so long a time that the relative real time difference iszero, RTD=0. In addition, it controls that each base station has thesame frame number or that adjacent base stations have only a minor phasedifference in their frame numbers. The calculation of the real timedifference and of tuning values is a continuous process demanding muchcalculation.

It was assumed in the examples described above, that the functionalityof location calculation is integrated in the base station controller.The Serving Mobile Location Centre SMLC may also be a separate unit inthe Base Station System BSS, whereby the measurement reports onmeasurements made by the location measurement unit are broadcast to itby way of the base station controller.

The SMLC has real-time information at all times about the network's realtime difference. Certain external applications, e.g. the authorities,may request information about the SMLC through the Gateway MobileLocation Centre GMLC, which allows access for a third party to thesubscriber's location information in the mobile network.

The drawings and the explanation relating to them are only intended toillustrate the inventive idea. The synchronisation of frame broadcastsat the air interface in accordance with the invention may vary indetails. The implementation of equipment needed in the system may alsovary. In the examples, the location measurement unit was connected bycables to the base transceiver station, but it may alternatively beintegrated directly into the base transceiver station. It can also belocated as a separate unit in such a way that it is in connection withthe base station through the air interface. However, this alternative isnot recommended, because the accuracy of clocks will suffer essentially.

It was said in the foregoing that the GPS receiver contained in thelocation measurement unit is used for receiving an accurate referenceclock in order to synchronise the base station clocks. However, it mayalso be used for location determination e.g. in order to check theposition of the location measurement unit. The correct and accurateposition information of the location measurement unit is important e.g.to the base station controller, since it calculates positioninformation. And it is also important to those other units of thenetwork, which calculate position information.

The location measurement unit supports the synchronisation and otherpositioning methods of the base station system, e.g. Enhanced ObservedTime Difference E-OTD. As the SMLC has real-time information on thenetwork's real time difference, the mobile station can be located by theE-OTD method in a non-synchronised network by utilising the informationof measurement reports broadcast by the location measurement units. TheSMLC hereby requests real time differences reported by the LMUs andcalculates the location of the mobile station with the aid of OTD valuesmeasured and reported by the mobile station.

One of the advantages of the synchronised network is that it alleviatesthe problem of decoding the signal broadcast by the base station, whichcontains base station identification information, and allows use of manysuch applications, which cannot be implemented in a non-synchronisednetwork. Dynamic allocation of frequencies and channels can be mentionedas an example.

The accurate global clock needed by base stations for synchronisation ofbroadcasts over the air interface may also be another clock than onebroadcast by satellites. The main thing is that the base stations cansynchronise reliably their own internal clock with a certain globaltime.

Although the invention was explained above mainly in connection with amobile system, synchronisation of frame broadcasts can be used also fora telecommunications system of some other kind, when frame broadcastsare broadcast and received over the air interface in thetelecommunications system. The method and system according to theinvention can be implemented either in the entire telecommunicationsnetwork or in a part of the telecommunications network, e.g. in such anarea only, which covers a certain town.

1. Method of synchronizing base transceiver stations in a mobilecommunications network, said method comprising: receiving a signalincluding an external system accurate clock; measuring a real timedifference (RTD) of broadcast signals from at least one base transceiverstation and from at least one adjacent base transceiver station;transmitting the measured RTD, whereby the base transceiver stations aresynchronized in accordance with the measured RTD; providing a frameclock and a base station frame number within the clock informationtransmitted by the location measurement unit to the at least one basetransceiver; and defining the frame clock with the aid of the externalclock.
 2. The method as defined in claim 1, further comprising:providing a plurality of base transceiver stations; connecting togethersaid plurality of base transceiver stations to form chained basedtransceiver stations; operating the location measurement unit as amaster clock of the chained base transceiver stations; and synchronizingthe chained base transceiver stations with identical clock information.3. Method of synchronizing base transceiver stations, the methodcomprising: receiving a signal including an external system accurateclock; transmitting a clock information of the signal to a basetransceiver station of at least one base station site; synchronizing thebase transceiver station in accordance with the clock information;determining whether the at least one base station includes aninoperative location measurement unit; synchronizing the basetransceiver station of the inoperative measurement unit using a locationmeasurement unit from an adjacent base station site; providing a frameclock and a base station frame number within the clock informationtransmitted by the location measurement unit to the at least one basetransceiver; and defining the frame clock with the aid of the externalclock.
 4. The method as defined in claim 3, further comprising:transmitting the clock information to a location calculation unit. 5.The method as defined in claim 4, further comprising: receiving, at thelocation measurement unit, frames transmitted by the at least one basetransceiver station; attaching to the frames a time stamp based upon theexternal system accurate clock; and transmitting the time stamped framestogether with the clock information.
 6. The method as defined in claim5, further comprising: transmitting the clock information and the timestamped frames from the at least one base transceiver station to thelocation calculation unit.
 7. The method as defined in claim 6, furthercomprising: reporting a real time difference from the locationcalculation unit to a base station controller, wherein the basetransceiver station controller manages and control a clock phasing ofthe at least one base station transceiver station having no locationmanagement unit; and transmitting a clock tuning value from the locationcalculation unit to the at least one base transceiver station having nolocation management unit.
 8. The method as defined in claim 4, furthercomprising: locating the location calculation unit in a base stationsystem.
 9. The method as defined in claim 3, further comprising:receiving, at the location measurement unit, the clock informationincluding the external accurate clock transmitted from a satelliteorbiting the earth.
 10. The method as defined in claim 3, furthercomprising: providing the location measurement unit as a componentwithin the at least one base transceiver station.
 11. The method asdefined in claim 3, further comprising: providing the locationmeasurement unit as a separate component in the at least one basestation.
 12. A mobile communications system comprising: a plurality ofmobile stations connected to at least one base station site; the atleast one base station site including at least one base transceiverstation having radio interface, wherein said radio interface having aframe structure formed by transmission frames and reception frames; abase station controller for controlling the frame transmissions of theat least one base transceiver stations; a location measurement unitconnected to the at least one base transceiver station; wherein thelocation measurement unit includes a receiver for receiving an externalsystem accurate clock, a reception device for receiving the transmissionframes of the base transceiver station of at least one adjacent basestation site, and a processor for processing clock information and thetransmission frames and for transmitting the clock information and thetransmission frames to the base transceiver; and a frame clock and abase station frame number within the clock information transmitted bythe location measurement unit to the at the least one base transceiverstation, wherein the frame clock is defined with the aid of the externalclock.
 13. A mobile communications system comprising: a receiverconfigured to receive a signal including an external system accurateclock; a location measurement unit configured to measure a real timedifference (RTD) of broadcast signals from at least one base transceiverstation and from at least one adjacent base transceiver station; atransmitter configured to transmit the measured RTD, whereby the basetransceiver stations are synchronized in accordance with the measuredand a frame clock and a base station frame number within the clockinformation transmitted by the location measurement unit to the at theleast one base transceiver station, wherein the frame clock is definedwith the aid of the external clock.
 14. A mobile communications systemcomprising: a receiver configured to receive a signal including anexternal system accurate clock; a transmitter configured to transmit aclock information of the signal to a base transceiver station of atleast one base station site; a first synchronizer configured tosynchronize the base transceiver station in accordance with the clockinformation; a controller configured to determine whether the at leastone base station includes an inoperative location measurement unit; asecond synchronizer configured to synchronize the base transceiverstation of the inoperative measurement unit using an locationmeasurement unit from an adjacent base station site; and a frame clockand a base station frame number within the clock information transmittedby the location measurement unit to the base transceiver station,wherein the frame clock is defined with the aid of the external clock.15. A location measurement apparatus, comprising: a receiver configuredto receive a signal including an external system accurate clock; ameasurement unit configured to measure a real time difference ofbroadcast signals from at least one base transceiver station and from atleast one adjacent base transceiver station; a transmitter configured totransmit the measured real time difference, wherein the base transceiverstations are synchronized in accordance with the measured RTD; and aframe clock and a base station frame number within the clock informationtransmitted by the measurement unit to the at least base transceiverstation, wherein the frame clock is defined with the aid of an externalclock.
 16. An apparatus, comprising: a transmitter configured totransmit a measured real time difference of broadcast signals from atleast one base transceiver station and from at least one adjacent basetransceiver station, wherein the base transceiver stations aresynchronized in accordance with the measured real time difference; and aframe clock and a base station frame number within the clock informationtransmitted by a location measurement unit to the at the least one basetransceiver station, wherein the frame clock is defined with the aid ofan external clock.
 17. An apparatus, comprising: a receiver configuredto receive a signal including an external system accurate clock; alocation measurement unit configured to measure a real time differenceof broadcast signals from two adjacent base transceiver stations; and aframe clock and a base station frame number within the clock informationtransmitted by a location measurement unit to at least one of the basetransceiver stations, wherein the frame clock is defined with the aid ofthe external clock.